A truly specific antibody to a particular tumor transplantation antigen might be expected to behave in the following ways. (a) Injected into an isogeneic host without tumor it would circulate in the blood with approximately the same half-life there as a normal gamma-globulin. (b) Injected into animals carrying this particular tumor it should localize preferentially only in this tumor tissue, or if tumor antigens are circulating in the blood, also in spleen and other tissues as antigen-antibody complexes. This research is directed at producing, in a study of carcinogen-induced tumors in an inbred strain of rats, 125I-labeled antibodies with these characteristics. Antibodies will be induced by allo- and isogeneic as well as xenogeneic immunization. Antibodies will be isolated from immune sera by absorption on and elution from membrane fractions prepared from tumor cells or from preparations of viable single tumor cell suspensions. Antibodies not specific for tumor will be removed by absorption with normal cells or cell fractions. In vivo screened 131I-labeled normal gamma-globulin will be used as tumor localization control. Preliminary studies indicate that 125I-antibodies showing substantial preferential tumor localization can be prepared by these methods. BIBLIOGRAPHIC REFERENCE: W.F. Bale, M.A. Contreras, M.J. Izzo, D. Della Penta and D.J. Buchsbaum, "Preferential in vivo Localization of 125I-labeled Antibody in a Carcinogen-Induced Syngeneic Rat Tumor." Progr. Exp. Tumor Res., Vol. 19, pp. 270-283, 1974.